1. Field of the Invention
My invention relates to processes and apparatus for storing solar energy in synthetic fuels, and more particularly to processes and apparatus for thus storing solar energy which employ molten gasification media.
2. Prior Art
The use of solar heating to produce synthetic fuel, thus storing solar energy in the synthetic fuel product, is taught in U.S. Pat. No. 3,993,458, issued to Michael J. Antal, Jr., on Nov. 23, 1976.
More particularly, Antal teaches that a solar heat fluidized reactant bed of char and organic material may be used to produce synthesis gas by gasification reactions. According to Antal, the synthesis gas produced by such gasification reactions can be used as a fuel, as a chemical feedstock, or as a raw material in the production of methanol. In the process of Antal, steam, carbon dioxide, or a mixture of these gases is used as the working fluid, and is heated in a tower top solar furnace. This working fluid is used to fluidize the reactant bed of char and solid organic material.
In the single structural embodiment of Antal, quartz windows are used to allow concentrated solar radiation to enter the bottom of a jacket surrounding the fluidized bed reactor and then to enter the bottom of the reactor.
In this embodiment, the working fluid is introduced into the jacket near the top portion of the jacket. The working fluid then flows to the bottom of the jacket, around the fluidized bed reactor. The bottom of Antal's jacket is supplied with a quartz window through which concentrated solar rays pass. These concentrated solar rays are used to heat the working fluid of Antal to a moderately high temperature, e.g., 700.degree. C. to 1100.degree. C. Antal's heated working fluid then enters the bottom of Antal's fluidized bed reactor through a second (perforated) quartz window and is used in the reactor to fluidize the reactant bed of char and organic material.
While the solar heated working fluid is thus entering Antal's fluidized bed reactor from the bottom, the organic material to be gasified is charged into the fluidized bed reactor from the top through a feed hopper and airlock system.
In accordance with the teachings of Antal, a catalyst of cobalt molybdate or NzHCO.sub.3 is mixed with the organic material to be gasified before it is charged to the fluidized bed reactor. The Organic material used in the process of Antal may, of course, be comminuted to a degree dependent upon the economics of the system as determined by those having ordinary skill in the art.
In the device of Antal, the organic feed material is pyrolyzed as it is heated by the working fluid. The products of the resulting pyrolysis reactions in the fluidized bed reactor of Antal include synthesis gas, e.g., a mixture of CO, H.sub.2, CH.sub.4, CO.sub.2, and H.sub.2 O, ashes, tars, oils, liquors, and char.
In the device of Antal, the solid char migrates to the lower section of the fluidized bed reactor where it is heated by the concentrated solar radiation passing through the imperforate window in the bottom of the jacket, and thence through the perforated window in the bottom of the reactor, and impinging upon the lower surface of the char.
The method and apparatus of Antal do not appear to be adapted to continuous processing of organic materials, since the catalyst of Antal is recovered by soaking the ash residue remaining after the gasification of the organic material in water, and no means is shown in the device illustrated in Antal for continuous removal of the ash residue.
Further, the fluidized bed reactor and surrounding jacket of the device of Antal must be insolated from below, rendering devices incorporating the teachings of Antal costly because high-strength support structures must be provided to elevate not only Antal's fluidized bed reactor and surrounding jacket, but also the associated feed stock hopper, air lock, and gas tight seals. In addition, these high-strength support structures for elevating the major portions of the Antal device must be constructed and arranged so that they do not substantially block the heliostat-directed solar radiation from reaching the quartz window located at the bottom of Antal's reactor jacket.
Moreover, the solid material handling means for charging organic material into Antal's refuse hopper must comprise an elevated structure extending at one end above the upper edge thereof, and yet at the same time must not substanially block the solar radiation directed by heliostats onto the quartz window at the bottom of Antal's reactor jacket. Again, as with the support structure for his fluidized bed reactor, etc., solid material handling means suitable for supplying carbonaceous material to Antal's refuse hopper is nowhere shown or described, however broadly, in Antal.
In addition, the gaseous gasification medium of Antal is inherently inferior in solar radiation absorptivity to molten solid gasification media, and thus processes for storing solar energy in synthetic fuels which employ molten solid gasification media will be characterized by higher efficiency of conversion of solar insolation into heat and will reduce capital equipment cost as compared with the method and apparatus of Antal.
Further, the gaseous gasification medium of Antal inherently possesses a much lower thermal conductivity per unit volume than do molten solid gasification media, and thus the efficiency of heat transfer to the carbonaceous material in devices embodying the invention of Antal will be less efficient than the corresponding heat transfer in devices employing molten solid gasification media, and the gasification reactor vessel of Antal will be considerably larger than optimum. The greater-than-optimum size of Antal's reactor will, of course, exacerbate the problems of elevatedly mounting Antal's reactor, etc., discussed hereinabove.
Yet further, the gaseous gasification medium of Antal must be passed through Antal's fluidized bed reactor at high gas velocities in order to maintain the carbonaceous material in the reactor in a fluidized state.
It follows, then, that large amounts of steam and/or carbon dioxide are needed to maintain the carbonaceous material in Antal's reactor in the fluidized state.
Since large amounts of steam and/or carbon dioxide are needed to maintain the fluidized state in the reactor of Antal, the amount of steam and/or CO.sub.2 in the raw product synthesis gas produced by the reactor of Antal will be much greater than the percentage of extraneous gases in the synthesis gas product of a plant employing a molten solid gasification medium.
Additionally, Antal does not teach method or means whereby to separate his raw synthesis gas from the carbonaceous material and char in the fluidized bolus or bed in his reactor. It appears evident from the teachings of Antal, however, and the fact that large amounts of Antal's gaseous gasification medium (steam and/or carbon dioxide) must be passed at high velocity through his fluidized bed, that a substantial portion of the carbonaceous material charged to Antal's reactor, and possibly a substantial portion of the char generated in Antal's reactor, will be emitted from Antal's reactor along with the raw synthesis gas product and ash emitted therefrom. This entrained carbonaceous material, and possibly char, will have to be separated from the ash and synthesis gas and returned to Antal's fluidized bed reactor if a high rate of conversion of carbonaceous material to synthesis gas is to be maintained in accordance with the teachings of Antal. However, neither method nor apparatus for thus separating carbonaceous material and char from the raw synthesis gas product of the reactor of Antal is taught anywhere in Antal. It follows that even were one having ordinary skill in the art to supply such a separation process for use in connection with the teachings of Antal, it might well be a costly process, substantially affecting the overall economics of the Antal process.
Finally, it should also be recognized that the gaseous gasification medium of Antal suffers from two additional deficiencies, viz., that the gasification medium does not, as do certain molten solid gasification media, serve as a catalyst for the gasification process taking place in the reactor, and that, unlike molten solid gasification media, the solar-absorbing properties of Antal's gaseous gasification medium cannot be enhanced by the addition thereto of suitable dopants.
Also, the gaseous gasification medium of Antal is very poorly adapted, if adapted at all, to employment in a multi-stage solar gasification process in which a separate working fluid is heated by insolation, and this working fluid in turn heats the gasification medium. Such a multi-stage process permits the selection of an optimum working fluid for direct insolation, i.e., a working fluid having characteristics such as good solar radiation absorptivity, chemical inertness, and stability at high temperatures.
Processes and apparatus for the gasification of carbonaceous materials into synthesis gas using molten solid gasification media are disclosed not only in prior U.S. patents but also in the technical literature. Among such prior U.S. patents are U.S. Pat. Nos. 3,708,270; 3,567,412; 3,252,773; and 3,916,617. In these patents, a portion of the carbon in the carbonaceous material feedstock being gasified is reacted with a limited amount of oxygen to provide the heat necessary to sustain the endothermic gasification reaction in the gasification reactor vessel. None of these patents makes any mention of the employment of solar radiation as an alternative to the reaction of oxygen with part of the carbon in the carbonaceous material to provide the necessary process heat. A fortiori, none of these patents teaches solar energy as the heat source in their disclosed carbonaceous material gasification processes. Typical technical articles disclosing prior art molten solid gasification medium processes for use in the gasification of carbonaceous materials into synthesis gas are found in the magazine Chemical Engineering Progress, March, 1973, Volume 69, No. 3. Of particular interest is the article in that journal entitled Kellog's Coal Gasification Process, commencing at page 31, and The COED Process Plus Char Gasification, commencing at page 43. Reference may also be had to the text New Energy Technology, by H. C. Hottel and J. B. Howard, 1971, The Massachusetts Institute of Technology. These technical publications like the above-cited U.S. patents, do not suggest the great advantages to be derived from the employment of solar energy as the heat sources in the described processes, nor teach the processes and apparatus for achieving these great advantages which are uniquely taught herein.